Cationic polymerization (for a recent review of the area, see: Comprehensive Polymer Science 1989, 3, 579 ff), in contrast to radical polymerization, is not inhibited by the presence of oxygen in air and proceeds after UV-initiation in the dark. Cationic polymerization is also complementary to radical polymerization with respect to polymerizable monomers. Thus, electron rich carbon-carbon double bonds (e.g. alkenyl ethers) are easily cationically polymerized while acrylate monomers are usually unreactive. Vinyl ethers however, do not homopolymerize under radical polymerization conditions. Epoxides is another commercially important class of monomers that polymerize readily under cationic conditions but are inert to radical polymerization.
The utility of cationic polymerization has strongly been tempered by the fact that previously developed cationic initiators have inferior technical properties such as unsatisfactory high initiation temperature and poor solubility in monomer blends and smelly decomposition products.
Strong proton acids (e.g. HClO.sub.4, HBF.sub.4) or Lewis acids (e.g. AlCl.sub.3, BF.sub.3) initiate cationic polymerization of for example vinyl ethers and epoxides. These acids have a very limited utility in a technical context, such as curing of a coating, mainly due to the immediate polymerization that occurs upon mixing initiator and monomer, i.e. the system has no "pot life".
This problem has been circumvented by developing "latent proton acids". They are structurally recognized by being aryl-substituted "onium-salts", e.g. sulfonium-, iodonium-, or arsonium-salts, with non-nucleophilic anions such as SbF.sub.6.sup.-, AsF.sub.6.sup.-, PF.sub.6.sup.-, and BF.sub.4.sup.-. These salts are stable, latent sources of the corresponding strong Bronsteds acid HSbF.sub.6, HAsF.sub.6, and HBF.sub.4 respectively, which are generated upon activation and initiate the polymerization. The salts are inactive until the activation occurs. A majority of this class of latent initiators require photochemical activation (irradiation by UV-light). (Belg. Pat. 828670, 1974; U.S. Pat. No. 3,981,897, 1976; Belg. Pat. 837782, 1970; Belg. Pat. 833472, 1976).
More recently, it has been shown that some sulfonium- and iodonium-salts can be thermally activated and utilized for the initiation of a cationic polymerization. Two methods for activation have been developed; redoxinitation (A. Ledwith, Polymer 1978, 19, 1217) and thermal initiation (Jap. Pat. 63.221.111, 1988, [CA 1989, 111, 40092y],; Jap. Pat. 63223002 1988 [CA 1989, 110, 173955h]; S. P. Pappas and L. W. Hill, J. Coating Technol., 1981, 53,43; S. P. Pappas and H. B. Feng, "Cationic Polymerization and Related Processes" ed. E. J. Goethals, Academic Press, New York, 1984; T. Endo and H. Uno, J. Polym. Sci., Polym. Lett. Ed., 1985, 23, 359; T. Endo and H. Arita, Makromol. Chem., Rapid Commun., 1985, 6, 137).
In common for these initiators is that the activation leads to a fragmentation, dissociation of the initiator molecule into decomposition products of lower molecular weight (e.g. sulfide) along with the initiation of a cationic polymerization, see figure. EQU R.sub.3 S.sup.+ X.sup.- .fwdarw.R.sub.2 S+R.sup.+ X.sup.- EQU R.sup.+ X.sup.- +M.fwdarw.RM.sup.+ X.sup.- EQU RM.sup.+ X.sup.- +nM.fwdarw.RM.sub.n M.sup.+ X.sup.-
The deficiency of previously developed initiators (PDI) can thus be summarized in three points, where the present invention in all three aspects furnishes considerable improvements:
1. PDI have poor solubility in monomers which generally have a lipophilic character. PA1 2. PDI yield low molecular decomposition products upon activation whose emission may cause enviromental problems. This is especially pronounced in the case of sulfonium salts where a low molecular sulfide is formed. PA1 3. Commercially available initiators are limited to photochemical activation. PA1 i) The sulfonium salt is a heterocyclic, arylsubstituted or with an arylring fused, compound. PA1 ii) The most stabilizing substituent at the sulfonium group is benzylic or substituted benzylic. PA1 n=an integer between 1 and 3 PA1 z=an integer between 0 and 3 PA1 y=an integer between 0 and 4 PA1 X=represents a group of the formula MY.sub.r (1) or the formula Q(2), PA1 R.sup.1 represents an alkyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, or an aryl group, PA1 R.sup.2 represents hydrogen, an alkyl, alkenyl, cycloalkenyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, or aryl group, all R.sup.2 being independent of each other, PA1 R.sup.3 represents hydrogen, an alkyl, alkenyl, cycloalkenyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, or aryl group, all R.sup.3 being independent of each other, PA1 R.sup.4 represents hydrogen, halogen, an alkenyl, for instance a vinyl group, a cycloalkenyl, an alkyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, an alkoxy or thioalkoxygroup, preferably C.sub.1 -C.sub.20, a hydroxyl- or alkyl(C.sub.1 -C.sub.12)terminated poly(alkyleneoxide) group with up to 10 alkyleneoxide units, an aryl group, an aryloxy or thioaryloxy group, PA1 R.sup.5 represents halogen, an alkyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, an alkoxy or thioalkoxy group, preferably C.sub.1 -C.sub.20, a hydroxyl- or alkyl(C.sub.1 -C.sub.12)terminated poly(alkyleneoxide) group with up to 10 alkyleneoxide units, an aryl group, an aryloxy or thioaryloxy group, wherein in structure I R.sup.4 or R.sup.5 (y=1-2) also can be the group ##STR6## R.sup.6 represents hydrogen, an alkyl, alkenyl, cycloalkenyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, or an aryl group, PA1 R.sup.7 represents hydrogen, an alkyl, alkenyl, cycloalkenyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, or an aryl group, PA1 A represents ##STR7## or a single bond, R.sup.8 represents hydrogen, an alkyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, a hydroxyl- or alkyl(C.sub.1 -C.sub.12)terminated poly(alkyleneoxide) group with up to 10 alkyleneoxide units, an aryl group, an aryloxy or thioaryloxy group, PA1 R.sup.9 represents hydrogen, an alkyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, a hydroxyl- or alkyl(C.sub.1 -C.sub.12)terminated poly(alkyleneoxide) group with up to 10 alkyleneoxide units, an aryl group, an aryloxy or thioaryloxy group, or PA1 R.sup.8 and R.sup.9 together form an aryl ring fused with the heterocyclic sulfonium ring, said aryl ring optionally being substituted with a group R.sup.10 which can be a halogen atom, a nitro group, an alkyl or cycloalkyl group, preferably C.sub.1 -C.sub.20, alkoxy or tihoalkoxy group, preferably C.sub.1 -C.sub.20, a hydroxyl- or alkyl(C.sub.1 -C.sub.12)terminated poly(alkyleneoxide) group with up to 10 alkyleneoxide units, an aryl group, an aryloxy or thioaryloxy group.
The activation of a thermal initiator involves a heterolytic cleavage of a carbon-sulfur bond to form the most stabilized carbocation. The activation temperature for an alkyl-substituted sulfonium salt strongly depends on the structure of the substituents. The activation temperature decreases if a more stabilized carbocation can be formed. Substituents of resonance stabilizing ability (e.g. benzylic and allylic) lower the temperature at which the cationic polymerization occurs. Electron-donating substituents (e.g. alkyl or alkoxy) in ortho or para positions at the benzylic group further decrease the activation temperature.
Besides controlling the initiation-temperature, the substituents have a strong influence on the solubility of the initiator-salt. Previously developed initiators have poor solubility in "solvent free" monomers such as epoxides, alkenyl ethers, or styrenes. This is due to the large polarity difference between the initiator and the monomer blend. Hydrophobic substituents such as longer n-alkyls can moderate the polar character of the initiator and improve its solubility properties in hydrophobic monomers.
Low molecular weight sulfides have a very strong and unpleasant smell even at very low concentration levels (ppm-levels). They are formed at the activation step and during the polymerization emission to the environment is very difficult to avoid. In addition, the remaining sulfide could at a later state migrate to the polymer surface and cause a smelly polymer film. It is therefore very important to structurally modify an initiator to avoid formation of low molecular weight sulfides.
The reaction scheme below illustrates the chemistry of a member of a previously developed initiator where tetrahydrothiophene is formed during the activation, followed by the initiation of a cationic polymerization of a vinyl ether. ##STR1##